Properties of HF (high frequency) band (also referred to as shortwave band) radio wave propagation are described below. In a HF band wireless communication system (HF data communication system), communication is generally carried out by using the propagation of radio waves through the ionosphere. Such usage of the propagation of radio waves through the ionosphere makes long distance, over-the-horizon communication possible, where the ionosphere is affected by the season, the time zone, the solar activity and the like, and therefore, the frequency that can cover the target communication area varies depending on the season and the time zone.
Since HF bands have the above-described properties, a number of frequencies are generally used in an HF data communication system so as to correspond the communication area that varies depending on the season and the time zone. However, the HF band frequencies are limited, and thus, it is difficult to cover the properties of the propagation of radio waves in all the HF bands simply by using a number of frequencies.